This invention relates to a stable, gel-like slurry having a pH of about 6.5 to about 8.5 containing about 30 to about 40 wt % (dry basis) of a monosodium cyanurate monohydrate. It also relates to the preparation of that slurry and its use in swimming pools and other water systems.
Cyanuric acid (CYA) is used in swimming pool water at levels from 25 to 150 ppm (parts per million by weight) to stabilize hypochlorous acid from sunlight decomposition. CYA is added to swimming pool water upon start up of the pool with fresh water or on a routine basis by those customers who use forms of chlorine other than chlorinated isocyanurates to maintain desired CYA concentration.
CYA is typically in a dry (containing 6 wt % or less water), granulated form. Typical means of application are to broadcast over the pool or to slurry the granules in water and pour the slurry down the pool skimmer. CYA is very slow to dissolve and can take several days to dissolve if broadcast into a pool. Until the CYA has been dissolved for at least 24 hours the pool operator must be careful not to backwash the filter as this would flush the CYA granules out of the filter before they can dissolve in the pool water. The CYA is a weak acid (pH about 3.0) and usually the pool water pH must be adjusted after it has been added.
Slurries of pure CYA untreated with caustic tend to settle and form hard cakes that are typically difficult to resuspend, making them a poor choice as a commercial slurry product. A typical CYA slurry in a half gallon vessel takes several minutes of constant shaking to resuspend the solids. If settled with vibration (such as occurs in transportation) the solids are even more difficult to resuspend. In some cases, after prolonged storage, the cakes become hard and can be removed from the container only by breaking the container open and physically agitating the cake.
Attempts have been made to improve the performance of CYA by reducing its particle size or by adding a dispersant, such as surfactant, but were not successful. The addition of polymers to pure CYA results in a stable slurry, but the dispersant properties of the slurry are usually diminished and an additional chemical is introduced into the pool.
It is well known that monosodium cyanurate (MSCY) has some advantages over CYA because it is more soluble and has a more neutral pH. But the MSCY products proposed have all been dry formulations that are slower to dissolve than slurries and require more capital to manufacture. U.S. Pat. No. 4,233,444, for example, discloses making monoalkali metal cyanurate continuously in a two reactor system. This patent was mainly for the production of chlorinated isocyanurates, but it mentions that monoalkali cyanurates can be used for stabilizing chlorine-containing water if the metal cyanurates are recovered by centrifuging and drying. This technology, as well as other technology for producing dry MSCY, is not believed to be commercially practiced due to the large capital requirements required to make it, including two reactors, a centrifuge, dryer, and possibly other solids handling equipment, such as either a mill or a compactor and screening equipment.